Generate a repeating mesh pattern without extra geometry - twisted cable/guitar string
Assuming Cycles, how about something like this :



This is simply a cylinder with a procedural texture based on a Wave Texture and a Radial Gradient Texture.



The Object coordinate is split into it's X,Y,Z coordinates and just the Z is used to feed into the Wave Texture X coordinate. Without any offset this would simply result in rings along the length of the mesh. Adjust the Scale of the Wave Texture to get the desired pitch. Adding in the output of the Radial texture skews the rings and this can be adjusted (using the Multiply node) to join each ring to the next to produce a continuous winding.

To double up the strands you can include maths nodes so that the 'dip' in the wave becomes a second strand - by subtracting 0.5 and then using Absolute to make the negative section positive.



Adjusting the pitch produces the following :



Use a metallic material and this might be reasonably close to your needs.

One limitation of this is that the 'turns' must be aligned with the texture space - in this case the Z axis of the Object coordinates - so the mesh must be aligned with the axis and cannot be curved. However, this limitation can be overcome by simply UV unwrapping the mesh and ensuring that the faces are all alligned and oriented correctly in UV space and this allows the mesh to take on any form while still maintaining the twists.

One way to achieve this is to create a curve or a path along the desired route of the 'string' and set the Curve Bevel settings to give it thickness.



Convert it into a Mesh (Alt-C).   



Go into Edit mode and select all faces (A) and select Mesh/UV Unwrap.../Follow Active Quads to unwrap the mesh. Open a UV/Image window and scale and align the unwrapped 'island' so that the narrow edge is just the same size as the UV image space as shown below.



With the UV Map scaled and positioned as shown, the Y coordinate can be used in place of the Radial Gradient in the previous material. The X coordinate will represent the length along the 'string'. This produces the following material and result :



Adjusting the Multiply nodes and the Wave Texture Scale will adjust the pitch and twistiness of the string.